Reference voltage generator for thermal processes having an exponential temperature-time characteristic

ABSTRACT

An improved arrangement is described for generating an exponential reference voltage for comparison in a suitable regulator with a value proportional to the measured temperature output of a thermal process having a desired exponential temperature-time characteristic. The generator includes an R-C circuit chargeable via a DC source wherein the time constant of the R-C circuit is chosen to equal the thermal time constant of the exponential thermal process. The voltage increase across the capacitor during charge or the voltage decrease across the capacitor during a subsequent discharge is employed selectively as a reference voltage for controlling, respectively, a rising and a falling exponential temperature-time characteristic of the thermal process. A cathode follower or other input buffer amplifier is coupled to the output of the capacitor to isolate the R-C circuit from the loading effect of the input of the regulator.

United States Patent Korntheuer June 25, 1974 REFERENCE VOLTAGE GENERATOR FOR THERMAL PROCESSES HAVING AN EXPONENTIAL TEMPERATURE-TIME CHARACTERISTIC [75] Inventor: Friedrich Korntheuer, Kapfenberg,

[2l] Appl. No.: 361,551

[30] Foreign Application Priority Data May 18, 1972 Austria 4331/72 [52] US. Cl. 307/149, 307/152 [51] Int. Cl. G05d 23/00 [58] Field of Search H05b/23/0O; 307/149, 152;

[56] References Cited UNITED STATES PATENTS 3.010,035 ll/l96l Calvert et al. 307/152 Primary Examiner-L. T. Hix Attorney, Agent, or FirmArthur O. Klein 5 7 ABSTRACT An improved arrangement is described for generating an exponential reference voltage for comparison in a suitable regulator with a value proportional to the measured temperature output of a thermal process having a desired exponential temperature-time characteristic' The generator includes an R-C circuit chargeable via a DC source wherein the time constant of the R-C circuit is chosen to equal the thermal time constant of the exponential thermal process. The voltage increase across the capacitor during charge or the voltage decrease across the capacitor during a subsequent discharge is employed selectively as a reference voltage for controlling, respectively, a rising and a falling exponential temperature-time characteristic of the thermal process. A cathode follower or other input buffer amplifier is coupled to the output of the capacitor to isolate the R-C circuit from the loading effect of the input of the regulator.

3 Claims, 2 Drawing Figures PAIEIHHIzs lam REFERENCE VOLTAGE GENERATOR FOR THERMAL PROCESSES HAVING AN EXPONENTIAL TEMPERATURE-TIME CHARACTERISTIC BACKGROUND OF THE INVENTION In the control of furnaces, e.g. those employed in the fabrication of steel products, it is often advantageous to employ a closed-loop temperature regulator of the furnace using a rising or falling temperature-time characteristic. In general, the regulation of such furnaces in this manner employs a themiocouple or other thermoelectric element which continually samples the temperature of the furnace and provides a proportional electrical signal, generally in the millivolt range. Such measured signal is continually compared at the regulator input with a reference electrical signal whose amplitude-time characteristic is mechanically varied in a manner that generally tracks the desired exponential variation of the thermal process. An error signal derived from such comparison is employed to vary the quantity of heat supplied to the furnace in such a manner as to minimize the error signal.

In presently known reference voltage generators for those applications, the required exponential variations of the amplitude characteristic is accomplished by mechanically driving, in a suitable manner, the wiper arm of a potentiometer whose input is coupled across a DC source. Unlike the far simpler case where the desired FIG. 1 is a curve showing a typical exponential temperature-time variation suitable for certain types of controlled furnaces; and

FIG. 2 is a block diagram of an arrangement in accordance with the invention for generating a reference control voltage proportional to the exponential temperature-time relationship of FIG. 1.

A closed-loop temperature regulation system of a type appropriate for instrumenting the temperaturetime characteristic of FIG. 1 is shown in FIG. 2. The quantity of heat to be applied to a furnace 7 to be regu lated is controlled by a conventional furnace regulator 6. The regulator is responsive to an error signal obtemperature characteristic of the furnace is to be linear (wherein the drive mechanism for the wiper arm can be instrumented in a straightforward way by a motor driven in accordance with velocity step function), the requirement of an exponential temperature variation requires, for the mechanically instrumented reference ,voltage generator, not only a motor but also mechanical motion-changing devices such as curved cams which are complicated and expensive.

SUMMARY OF THE INVENTION An improved arrangement for generating the required exponentially varying reference voltage for controlling thermal processes without the necessity of complex mechanical components is provided by the instant invention. In one embodiment, the input of an R-C electric circuit chosen to have an RC time constant corresponding to the thermal time constant of the desired temperature-time characteristic of the furnace is coupled across a DC source. The output voltage vari BRIEF DESCRIPTION OF TI-IEDRAWING The invention is further set forth in the following detailed description taken in conjunction with the appended drawing in which:

tained by a comparison, at the regulator input, between a first electrical quantity representative of the actual temperature of the furnace at each instant, and a second electrical quantity representative of the desired temperature (in accordance with the curve of FIG. 1) at that instant. Accordingly, such second quantity ideally has an amplitude that decays exponentially, with a time constant corresponding to Ta.

The first quantity mentioned above is the voltage output of a thermocouple or other thermoelectric element 8, which is disposed in thermal communication with the furnace 7. The second quantity is the voltage output of an exponential voltage generator 10.

In accordance with the invention, the furnace control system shown in FIG. 2 employs, as the reference generator 10, an all-electric arrangement including a waveshaping R-C circuit 3 having a resistor 11 and a capacitor 12. The input of the circuit 3 is connected to the output of a DC voltage source 1 via a switch 2. (The source 1 may be made adjustable if desired). The output of the circuit 3 is taken from the capacitor 12.

It will be recognized that once the capacitor 12 has been charged to the voltage of the source 1 by closure of the switch 2, a subsequent discharge of the capacitor (initiated, eg by closure of a switch 13 which may be controlled by the furnace regulator 6), will cause the capacitor voltage to decrease exponentially from its initial value with a time constant Tf= RC.

Consequently, the desired reference control voltage for the furnace 6 may be obtained from the capacitor output by so selecting the values of the resistor 11 and the capacitor 12 that Theoretically, the capacitor output voltage could be applied directly to the input of the regulator as the desired reference voltage without intervening circuitry. However, in order to prevent the input resistance of the regulator from loading the output of the capacitor 12 and thereby shifting the effective value of the time constant Tf from the desired value Ta, a bufier amplifier 4 having a high input impedance (e.g. an impedance that preferably is at least 10 times as large as the'resistance of resistor 11) is coupled to the output of the circuit 3. Such amplifier 4 may typically be a cathode or an emitter follower.

Moreover, since the output voltage from the amplifier 4 may typically be at least one order of magnitude greater than the millivolt range output of the thermcouple 8 to which the reference voltage is to be compared, an adjustable voltage divider 5 of suitable reduction ratio may be provided as shown between the output of the amplifier 4 and the associated input of the regulator 6.

In the foregoing, the invention has been described in connection with a preferred arrangement thereof. Many modifications and variations will now occur to those skilled in the art. For example, if the furnace 7 is to have an exponentially decreasing temperature-time characteristic, the corresponding exponentially increasing reference control voltage can be obtained from the arrangement of FIG. 2 by extracting the output voltage from the capacitor 12 during the charging thereof from the source 1 via the switch 2. Accordingly, it is desired that the scope of the appended claims not be limited to the specific disclosure herein con-v time is converted to a first proportional electrical signal which is compared continually at the input of a furnace regulator with a second electrical reference signal whose amplitude-time characteristic is proportional to the desired exponential characteristic of the process to generate an error signal employed by the regulator to control the quantity of heat supplied to the process, an improved arrangement for generating the referenced signal, which comprises:

an electrical wave shaping path including a resistor and a capacitor in series, the values of the resistor and the capacitor being chosen so that the resulting time constant of the path is equal to Ta:

means for coupling the input of the path to a DC source; and means for coupling the output of the capacitor to the input of the regulator to constitute the reference voltage.

2. A system as defined in claim 1, in which the coupling means comprises a high input impedance buffer amplifier.

3. A system as defined in claim 2, in which the coupling means further comprises a voltage divider disposed between the output of the buffer amplifier and the input of the regulator. 

1. In a closed-loop system for controlling a thermal process whose desired temperature-time characteristic is an exponential function having a time constant Ta, wherein the actual temperature of the process at any time is converted to a first proportional electrical signal which is compared continually at the input of a furnace regulator with a second electrical reference signal whose amplitude-time characteristic is proportional to the desired exponential characteristic of the process to generate an error signal employed by the regulator to control the quantity of heat supplied to the process, an improved arrangement for generating the referenced signal, which comprises: an electrical wave shaping path including a resistor and a capacitor in series, the values of the resistor and the capacitor being chosen so that the resulting time constant of the path is equal to Ta: means for coupling the input of the path to a DC source; and means for coupling the output of the capacitor to the input of the regulator to constitute the reference voltage.
 2. A system as defined in claim 1, in which the coupling means comprises a high input impedance buffer amplifier.
 3. A system as defined in claim 2, in which the coupling means further comprises a voltage divider disposed between the output of the buffer amplifier and the input of the regulator. 